The obvious solution would appear to be for those wanting Fast Broadband to have their circuits diverted at the exchange to run the extra 40m to the new FTTC cabinet. Looking at the works done for the new cabinet suggests that existing EO lines have been diverted in exactly that way.
Just to be clear - when you say "at
the exchange" - the EO diversion will be just outside the exchange, rather than within it - the technical regulations (known as the ANFP) don't want the VDSL2 frequencies within the exchange building.
Have I missed some technical subtlety that makes it impossible to route a circuit through two cabinets?
You might have missed a technical subtlety. Not about routing through 2 cabinets as such (secondary cabinets already exist in the network), but about whether it is feasible to divert your cable at all. Not about copper, but about insulation and protection.
The subtlety stems from the fact that E-side cables are different from D-side cables.
E-side cables, feeding from exchange to cabinet, have lots of pairs, tend to be older, and can even be insulated internally with paper. They are often protected from water incursion using air pressure fed from the exchange. Keeping water out is vital, so there is a great desire to not break into these cables at all. The oldest E-side cables can be lead-covered - so any work requires the services of a skilled lead jointer who can re-seal afterwards. Rare.
Diverting an E-side cable means the portion between the diversion point and the existing cabinet will no longer be protected from water ingress. And may be made of a material that sucks water in.
D-side cables, feeding from cabinet to poles, have fewer pairs, and are more likely to have been replaced - so nowadays tend to be gel-filled for water protection. These are so much easier to break into, as the gel automatically keeps water out.
EO lines are a strange mix, but are much less likely to be large bundles of air-protected cable. That means there is a reduced risk from breaking into an EO bundle, and diverting it to a new cabinet.
If your E-side cable is constructed this way, the last thing BT would want to do is to break into the cable - as this will remove any remaining protection as it runs from (just outside) the exchange to the PCP.
That means they would need to replace the entire length of the E-side cable to the cabinet, using a gel-filled alternative. Definitely not cheap.
Quite how much this applies to tiny village exchanges is another matter.
Or are the non-FTTC customers simply the victims of an Openreach policy which conveniently leaves angry people desperate for Fast Broadband, and so builds up political pressure for Openreach to be given a further round of public subsidy for conversion of remaining cabinets?
Whoever is left out will have a cadre of angry people - it doesn't need a policy from Openreach for this to happen. But there are probably more of them who think the political pressure should be to NOT give the money to Openreach.